Field of the Invention
This disclosure relates to medical instruments and more particularly to bone anchor systems for spine treatments, wherein the anchors can be configured for revision following a treatment interval.
Description of the Related Art
Thoracic and lumbar spinal disorders are a major socio-economic concern in the United States affecting over 70% of the population at some point in life. Low back pain is the most common musculoskeletal complaint requiring medical attention, and is the fifth most common reason for all physician visits. The annual prevalence of low back pain ranges from 15% to 45% and is the most common activity-limiting disorder in persons under the age of 45.
Degenerative changes in the intervertebral disc often play a role in the etiology of low back pain. Many surgical and non-surgical treatments exist for patients with degenerative disc disease (DDD), but often the outcome and efficacy of these treatments are uncertain. In current practice, when a patient has intractable back pain, the physician's first approach is conservative treatment with the use of pain killing pharmacological agents, bed rest and limitations on spinal segment motion. Only after an extended period of conservative treatment will the physician consider a surgical solution, which often is spinal fusion of the painful vertebral motion segment. Fusion procedures are highly invasive procedures that carry surgical risk as well as the risk of transition syndrome described below, wherein adjacent levels will be at increased risk for facet and discogenic pain.
More than 150,000 lumbar and nearly 200,000 cervical spinal fusions are performed each year to treat common spinal conditions such as degenerative disc disease and spondylolisthesis, or misaligned vertebrae. Some 28 percent are multi-level, meaning that two or three vertebrae are fused. Such fusions “weld” unstable vertebrae together to eliminate pain caused by their movement. While there have been significant advances in spinal fusion devices and surgical techniques, the procedure does not always work reliably. In one survey, the average clinical success rate for pain reduction was about 75%; and long time intervals were required for healing and recuperation (3-24 months, average 15 months). Probably the most significant drawback of spinal fusion is termed the “transition syndrome” which describes the premature degeneration of discs at adjacent levels of the spine. This is certainly the most vexing problem facing relatively young patients when considering spinal fusion surgery.
Many spine experts consider the facet joints to be the most common source of spinal pain. Each vertebra possesses two sets of facet joints, one set for articulating to the vertebra above and one set for the articulation to the vertebra below. In association with the intervertebral discs, the facet joints allow for movement between the vertebrae of the spine. The facet joints are under a constant load from the weight of the body and are involved in guiding general motion and preventing extreme motions in the trunk. Repetitive or excessive trunkal motions, especially in rotation or extension, can irritate and injury facet joints or their encasing fibers. Also, abnormal spinal biomechanics and bad posture can significantly increase stresses and thus accelerate wear and tear on the facet joints.
Recently, technologies have been proposed or developed for disc replacement that may replace, in part, the role of spinal fusion. The principal advantage proposed by complete artificial discs is that vertebral motion segments will retain some degree of motion at the disc space that otherwise would be immobilized in more conventional spinal fusion techniques. Artificial facet joints are also being developed. Many of these technologies are in clinical trials. However, such disc replacement procedures are still highly invasive procedures, which require an anterior surgical approach through the abdomen.
Clinical stability in the spine can be defined as the ability of the spine under physiologic loads to limit patterns of displacement so as to not damage or irritate the spinal cord or nerve roots. In addition, such clinical stability will prevent incapacitating deformities or pain due to later spine structural changes. Any disruption of the components that stabilized a vertebral segment (e.g., disc, facets, ligaments) decreases the clinical stability of the spine.